1. Field of the Invention
The present invention relates to a method for manufacturing an extruded article made of metal or polymer such as plastic and changing in cross-section along a direction of an extrusion axis. The present invention also relates to an apparatus for extruding such an extruded article.
2. Description of Related Art
A pipe member and a shaped member made of metal or plastic have been widely used for various industrial fields for the purpose of lightening the member and/or the whole product since such a member is high in flexural stiffness and torsional stiffness for its weight. In many cases, such a member is manufactured by an extrusion method which is excellent in manufacture efficiency.
However, an extruded article has the same cross-section along the entire length thereof since the extruded article is formed by extruding a billet through a bearing hole of a die having a fixed cross-section. Therefore, in the event that an extruded article is required to have different strength at longitudinally different portions so as to withstand different external force such as bending moment at different portions, the extruded article is designed to have the same cross-section along the entire length thereof so as to withstand the maximum external force.
Consequently, the extruded article includes a portion unnecessarily high in strength such as an unnecessary thick portion, which raises the material cost of the whole extruded article and the weight thereof.
Furthermore, in the event that an extruded article is used as a part of a product and that the end portion of the extruded article is to be joined to another member by welding or the like, the cross-sectional area of the end portion of the extruded article may sometimes be required to have a cross-sectional area larger than that of the remaining portion thereof so as to enhance the joint strength. However, it was impossible for a conventional manufacturing method to cope with the aforementioned requirements.
An object of the present invention is to provide a method for manufacturing an extruded article changing in cross-section along a direction of an extrusion axis.
Another object of the present invention is to provide an apparatus for manufacturing such an extruded article.
In order to attain the aforementioned object, according to a first aspect of the present invention, a method for manufacturing an extruded article which has at least one hollow portion and changes in cross-section along a direction of an extrusion axis, includes the step of advancing a punch to extrude a billet while controlling a movement of a mandrel for defining an inner periphery of the hollow portion relative to a die for defining an outer periphery of the extruded article.
It is preferable that the mandrel is controlled so as to advance or retreat in the direction of the extrusion axis while the punch is advancing.
The mandrel may be controlled so as to retreat until a tip end of the mandrel is located behind a bearing hole of the die while the punch is advancing.
The mandrel may have a tip end portion asymmetrical to a rotary axis of the mandrel, and the mandrel may be controlled so as to rotate about the extrusion axis or an axis parallel to the extrusion axis while the punch is advancing.
A moving speed of the mandrel may be controlled independently of a moving speed of the punch.
The mandrel may be controlled so as to move continuously or intermittently.
The aforementioned method for manufacturing an extruded article may further include the steps of applying a pressure to the billet in a state in which a bearing hole of the die is closed to fill the bearing hole with the billet and releasing a closing of the bearing hole prior to the step of extruding the billet by the mandrel and the punch.
According to a second aspect of the present invention, an apparatus for manufacturing an extruded article which has at least one hollow portion and changes in cross-section along a direction of an extrusion axis, includes a container in which a billet is loaded, a punch for applying a forward pressure to the billet, a mandrel for defining an inner periphery of the hollow portion and a die for defining an outer periphery of the extruded article. The mandrel is capable of moving relative to the die, whereby the extruded article is formed by advancing the punch to extrude the billet while controlling a movement of the mandrel relative to the die.
The mandrel may be capable of advancing and/or retreating relative to the die.
Furthermore, the mandrel may have a tip end portion asymmetrical to a rotary axis of the mandrel, and the mandrel may be capable of rotating about the extrusion axis while the punch is advancing.